The invention relates to a catalyst carrier a gas permeable support and with a porous surface coating of high silica content. The invention furthermore relates to a method for producing a catalyst support from synthetic, amorphous silica particles with an SiO.sub.2 content of more than 99%, by weight, preferably more than 99.5%, the silica particles being made into a plastic dough and the dough being shaped into a greenware and then sintered at high temperature.
Catalysts are used in the chemical and pharmaceutical industry, among other industries, in the manufacture of fine chemicals and for cleaning exhaust gases in industrial plants as well as in gas, gasoline and Diesel engines. The application determines the choice of the materials and the form of the catalyst support. Usually it consists of a support body of a chemically inert material, onto which the catalytically active substance is applied. In general, the greater the surface area, the better the catalyzing action. Therefore support bodies of great specific surface area are preferred. Often, however, the materials suitable as support material do not have the required great specific surface area. In general, the support body is then provided with a surface coating referred to as a "wash coat," which consists of a material of great specific surface area.
A catalyst support of this kind is disclosed in U.S. Pat. No. 3,804,647. In the catalyst support described therein, on the surface of a monolithic, gas-permeable support body, which can consist of a ceramic, glass-ceramic or vitreous composition, a slip coat is deposited, which contains a finely ground, porous borosilicate glass with a content by weight of 96 percent SiO.sub.2. After it is applied the slip coat is dried and sintered on tightly at about 800.degree. C. It then serves as a wash coat to increase the specific surface area of the catalyst support. The sintering temperatures and sintering time of the slip coat are coordinated such that any melting of the porous borosilicate glass content will be prevented so as to retain its high specific surface area insofar as possible.
The catalyst support thus prepared can be made with a honeycomb structure and used in conjunction with a catalytic metal coating as an exhaust gas catalyst up to temperatures of about 870.degree. C. However, due to the relatively low resistance of the borosilicate glass content to creeping, the specific surface area of the surface layer quickly decreases, and with it the catalytic effect. On account of its layered construction of materials of different thermal expansion, such catalyst supports have a relatively low strength; especially where high temperatures are used the surface layers spall off. It has also been found that the acid resistance of the support and of the surface layers of the known catalyst supports is insufficient in many applications, such as the cleaning of Diesel motor exhausts containing sulfur dioxide.
Wash coats are widely used which are made on an Al.sub.2 O.sub.3 basis. These, however, have the disadvantage that the Al.sub.2 O.sub.3 undergoes a phase conversion at about 700.degree. C. which results in a reduction of the specific surface area such that the specific surface area of the coating tends toward zero. Thus the catalytic activity is also lost.
Basically, for the application of the wash coat, a number of steps are necessary, each requiring a quality assurance step. The process for the manufacture of such catalyst supports is thus expensive and complicated.
German Patent Application DE-A1 39 12 504 discloses a method for preparing a catalyst support in the form of compacts, in which pyrogenically made silicon dioxide particles are homogenized with urea, methyl cellulose, aluminum stearate and/or magnesium stearate as well as graphite, with the addition of water. The dough thus prepared is then dried at a temperature of 80.degree. C. to 120.degree. C. and again crushed to powder. This powder is then pressed to form compacts and heat treated for a period of 0.5 to 8 hours at a temperature of 400.degree. C. to 1200.degree. C.
By the process disclosed in DE-A1 39 12 505 catalyst supports can be made in the form of pill-like compacts in, for example, cylindrical, spherical or annular shapes, with an outside diameter of 2 mm to 15 mm.
Pyrogenically made silicon oxides are characterized by extremely fine particles and an accordingly low space-filling quality. Due to this fineness, shaping them into the known catalyst compacts of simple geometrical shape presents a number of difficulties. The production of catalyst supports with a filigree structure is not possible by the known method.